During the current process for producing the conventional lithium ion battery with lithium cobaltate, lithium manganate, lithium nickel-cobalt-manganate (triple-element material), etc. as the active material for the positive electrode, PVDF (polyvinylidene fluoride) as binder and the organic solvent NMP (N-methyl pyrrolidone) as solvent are used when the positive electrode is typically prepared by a certain manufacturing process. However, when lithium iron phosphate is used as the active material of the positive electrode, the low tap density and the high surface area of lithium iron phosphate (compared to lithium cobaltate or lithium manganate) result in poor processability, serious de-powder phenomenon and low yield when PVDF as binder is used to prepare the positive electrode, which is the technical bottle-neck in the production of lithium ion battery with lithium iron phosphate. Meanwhile, PVDF requires organic solvents (mainly N-methyl pyrrolidone) to dissolve, thus pollution, recovery and treatment of the solvent require high production cost.
Compared with non-water soluble binder, water-based binder has become an important development trend in binder industry because of its good characteristics such as no organic solvent release, no environmental pollution, low cost, non-combustible, safety in using and the like. For instance, Chinese publication number CN1960040A discloses the use of the acrylic polymer as binder, and Chinese publication number CN1485940A discloses the water-based binder prepared by mixing CMC, etc. as thickening agent, styrene-butadiene rubber emulsion, etc. as binder and water soluble silane coupling agent, etc. as additive. So far, the conventional water-based binder includes phenol aldehyde resins, amine resins, polyacrylates, rubber latex, ethylene-vinyl acetates, etc. This binder generally contains hydrophilic groups, and the research reveals that the hydrophilic groups and water solvent have tremendous influence on the performance of lithium ion battery. The main reason is that it is hard to remove the water molecules remained in the electrode of lithium battery when using this binder. During the process of charge-discharge after assembling lithium battery, the water molecules will decompose to produce gas at a certain voltage, and further react with the electrolyte in lithium battery, which causes the problems on gas-expansion of lithium battery and too fast degradation of capacity. Therefore, it is the key for lithium ion battery to obtain success in commercialization that how to eliminate the influence of hydrophilic groups and water solvent on the performance of the battery.
At the same time, because of the poor conductivity of the positive electrode active material, such as lithium cobaltate, lithium manganate, lithium ferrous phosphate and so on, it is necessary to add some conductive agents, such as graphite and conductive carbon black, to ensure the conductivity of the electrode. Therefore, when choosing a binder, it is also very important to select a binder without influence on the conductivity of the electrode.
Due to its good using effect, a lithium iron phosphate electrode has an extensive application in making lithium battery. However, the current manufacturing process for lithium iron phosphate electrode still needs to be improved in the aspect of forming electrode and the like, and it is necessary to investigate deeply on selecting a good binder for preparing lithium iron phosphate electrode. According to the previous research results, the better effects can be obtained when an arabic gum is used as the binder in manufacturing electrode, but the prepared electrode still has the defects of surface smoothness and de-powder phenomenon. Therefore, it is necessary to further develop a solution to improve electrode smoothness and to decrease de-powder phenomenon without decreasing the overall performance of the electrode.